Isosbestic Research Articles

Isosbestic Point in Optical Mapping; Theoretical and Experimental Determination With Di-4-ANBDQPQ Transmembrane Voltage Sensitive Dye.

Optical mapping methods utilize fluorescence dyes to measure dynamic response of cardiac tissue such as changes in transmembrane potential (Vm). For the commonly used Vm sensitive dyes, a dye absorption and emission spectra shift as Vm changes. Signals relevant to Vm are calculated as a relative fluorescence change with respect to the fluorescence baseline. The amplitude of the change depends on the long-pass (LP) filter cut-on wavelength, placed on the sensor side, and the excitation wavelength. An excitation wavelength near the absorption peak, termed the isosbestic point, results in minimal absorption coefficient change as absorption spectra shifts. Consequentially the fluorescence intensity virtually does not change, when fluorescence across the entire emission spectra is measured, irrelevant of Vm changes. In this study we experimentally determined the isosbestic point for a near infrared dye Di-4-ANBDQPQ. We then present a theoretical study examining the dye linear or non-linear response as the fractional fluorescence change of Vm change, due to emission spectra shift and amplitude change, over a range of excitation wavelengths and LP filters. Linear "optical" response is important to quantify certain aspects of cardiac dynamics such as the action potential (AP) shape and duration, especially when studying drug effects and dynamical substrates for arrhythmia development.

Vibrational spectra of methylammonium iodide and formamidinium iodide in a wide temperature range

Transmission infrared, Attenuated Total Reflectance (ATR) and Raman spectra of crystalline methylammonium iodide (MAI) and formamidinium iodide (FAI) in the temperature interval starting from –170 ºC to 200 ºC were studied. The spectra recorded in the region from 4000 to 500 cm–1 enabled resolving the ambiguities associated with the origin of some bands. For the first time a complete and detailed vibrational investigation and assignment of the IR spectra of these compounds based on the differences in the temperature dependent IR spectra for all phases, including the metastable ones, have been made. The findings support the already established crystal structure of the phases for both compounds. The correlation between the overtones and fundamental modes has been confirmed based on the temperature induced isosbestic point.

UV-Vis Absorption Properties of New Aromatic Imines and Their Compositions with Poly({4,8-bis[(2-Ethylhexyl)oxy]Benzo[1,2-b:4,5-b']Dithiophene-2,6-diyl}{3-Fluoro-2-[(2-Ethylhexyl)Carbonyl]Thieno[3,4-b]Thiophenediyl}).

In this paper, four new aromatic imines containing at least one thiazole-based heterocycle were analyzed in detail by UV–Vis spectroscopy, taking into consideration their chemical structures and interactions with PTB7, a known polymeric electron donor widely used in bulk heterojunction organic solar cells. It is demonstrated that the absorption spectra of the investigated active compositions can be modified not only by changing the chemical structure of imine, but also via formulations with PTB7. For all investigated imines and PTB7:imine compositions, calibration curves were obtained in order to find the optimum concentration in the composition with PTB7 for expansion and optimization of absorption spectra. All imines and PTB7:imine compositions were investigated in 1,2-dichlorobenzene by UV–Vis spectroscopy in various concentrations, monitoring the changes in the π–π* and n–π* transitions. With increasing imine concentrations, we did not observe changes in absorption maxima, while with increasing imine concentrations, a hypochromic effect was observed. Finally, we could conclude that all investigated compositions exhibited wide absorptions of up to 800 nm and isosbestic points in the range of 440–540 nm, confirming changes in the macromolecular organization of the tested compounds. The theoretical calculations of their vibration spectra (FTIR) and LUMO–HOMO levels by Density Functional Theory (DFT) methods are also provided. Finally, IR thermal images were measured for organic devices based on imines and the imine:PTB7 composite.

Multiresponse Kinetic Modeling of Heat-Induced Equilibrium of β-Carotene cis-trans Isomerization in Medium-Chain Triglyceride Oil.

The kinetics and mechanism of the stepwise cis-trans isomerization reactions of all-trans-β-carotene dissolved in MCT (medium-chain triglyceride) oil at temperatures in the range of 80-160 °C have been analyzed using multiresponse modeling. Quantitation of the cis-isomers was performed using HPLC-DAD and quantitation at the reaction isosbestic point at 421 nm. Multiresponse kinetic modeling using the Bayesian criterion was initially performed at 120 °C to determine the best model. Subsequently, the reparametrized Arrhenius equation was used to calculate the activation energies of all reactions. The equilibrium constants for the individual isomerization reactions were determined from the rate constants and the final product distributions. The enthalpies and entropies of the isomerization reactions were determined from the temperature dependence of the equilibrium constants. The 13-cis and 13,13'-di-cis isomers were found to be the fastest formed isomers followed by the 9-cis, 9,13-di-cis, and 13,15-di-cis isomers, where the latter was found to be formed from 13-cis and not the 15-cis isomer. The relative free energies of the β-carotene isomers were determined as all-trans < 13-cis < 9-cis < 13,13'-di-cis < 9,13-di-cis ≈ 15-cis < 13,15-di-cis. The entropic contribution of each reaction was found to play an important role in the ordering. It is concluded that the β-carotene system is quite labile at temperatures ranging from 80 to 160 °C and resulting in equilibrium distributions of the cis-trans isomers.

UV/vis spectrophotometric determination of slow equilibrated N(1)-H missing deprotonation constant of a pyrimidine and thiopyrimidine: The final situation of the four pKa values.

The missing acidity constant of recently synthesized 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-one (I) and 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-thione (II) are investigated as a function of time at pH values from 1.0 to 3.20 for I and from 1.0 to 4.06 for II using UV/vis spectroscopic analysis of their aged solutions, at a temperature of 25 ± 0.1 °C. In this study, a novel time-dependent isosbestic point method was developed to confirm the presence of any weak acid - base equilibrium and the detection of related equilibration time. The missing acidity constants for each of I and II were determined in 5.0% v/v aqueous methanol. Plausible acid base equilibrium mechanism for the missing acidity constant was presented for each of the compounds based on the extensive experimental UV/vis data. In this context, all the acid base equilibria for each of the regarding compounds were completely revised through different pH values.

Interaction of AnVI with Isothiocyanate Ion in Water‐Organic Media (An = U, Np, Pu)

Behavior of UVI, NpVI and PuVI in water‐acetonitrile solutions was studied spectrophotometrically with the successive addition of the polar organic ligands (dimethyl sulfoxide or hexamethylphosphoric triamide) and the NCS– ion. The detected spectral effects – changes in the absorption intensity, bathochromic shifts in the absorption bands, the absence of isosbestic points, a change in the color of the solution – indicate complex competitive processes occurring in the studied solutions. In the case of NpVI, its partial reduction to NpIV by NCS– ion is observed. Solid UVI complex, [UO2(HMPA)2(NCS)2], was isolated, its crystal structure was determined using X‐ray diffraction. In contrast to known AnO22+ compounds with the NCS– ion, this complex exhibits tetragonal bipyramidal environment of the U atom. [UO2(HMPA)2(NCS)2] is also characterized by UV/Vis, IR and luminescence spectroscopy.

Photooxidation Reactions of Cyclometalated Palladium(II) and Platinum(II) Complexes.

New C,N,S-cyclometalated palladium(II) and platinum(II) complexes have been synthesized and their structural, electrochemical, and photochemical properties examined. The blue color of these complexes in solution changed to yellow under visible-light irradiation. By measurement of the absorption spectra for quantifying changes in color, isosbestic points for each complex clearly indicated the presence of only two species responsible for the change of color. X-ray analysis revealed that the visible-light-induced yellow species were S-oxygenated sulfinato complexes. Photosensitized generation of singlet oxygen (1O2) was confirmed by the direct detection of singlet oxygen luminescence at 1275 nm. The present cyclometalated palladium(II) and platinum(II) complexes are efficient photosensitizers of singlet oxygen, which rapidly reacts with coordinating sulfur atoms.

Synthesis of Strongly Fluorescent Imidazole Derivatives: Structure Property Studies, Halochromism and Fluorescent Photoswitching.

New series of methoxy and hydroxyl group substituted triphenylamine (TPA)-imidazole fluorescent molecules (5-(diphenylamino)-2-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol (1), 5-(diphenylamino)-2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenol (2), 5-(diphenylamino)-2-(4,5-diphenyl-1H-imidazol-2-yl)phenol (3), 5-(diphenylamino)-2-(1,4,5-triphenyl-1H-imidazol-2-yl)phenol (4), N-(3-methoxy-4-(1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-N-phenylbenzenamine (5), N-(3-methoxy-4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-N-phenylbenzene amine (6), and N-(3-methoxy-4-(4,5-diphenyl-1H-imidazol-2-yl)phenyl)-N-phenylbenzenamine (7)) have been synthesized that exhibited strong solution fluorescence and molecular structure and conformation controlled fluorescence photoswitching, solid state fluorescence and halochromism. Hydroxyl substituted molecules (1-4) showed moderate to strong fluorescence in solution depend on solvent polarity and very weak solid state fluorescence. Methoxy substituted molecules (5-7) displayed strong fluorescence both in solution and solid state. Solid state structural studies revealed strong intramolecular H-bonding in the crystal lattice. Interestingly, highly twisted structure (6) showed rare light induced reversible fluorescence switching in CHCl3. The observation of isobestic point in time dependent fluorescence photoswitching studies indicated structural isomer conversion. Further, acid sensitive imidazole nitrogen has been made use to demonstrate solid state fluorescence switching via halochromism. Thus the present studies attempted to develop new fluorescent molecules and establish structure-property relationship for designing fluorescence switching materials. Graphical Abstract Molecular structure controlled solid state fluorescence, halochromism and a rare fluorescence photoswitching in chloroform have been observed with triphenylamine-imidazole derivatives.

Spectroscopic Characterization of μ-η1:η1-Peroxo Ligands Formed by Reaction of Dioxygen with Electron-Rich Iridium Clusters.

Although oxygen is a common ligand in supported metal catalysts, its coordination has been challenging to elucidate. We now characterize a diiridium complex that has been previously shown by X-ray diffraction crystallography to incorporate a μ-η1:η1-peroxo ligand. We observe markedly enhanced intensity at 788 cm-1 in the Raman spectrum of this complex, which is a consequence of bonding of the peroxo ligand but does not shift upon 18O labeling. Electronic structure calculations at the density functional theory level suggest that this increase in Raman intensity results from bands associated with rocking of CH2 substituents directly attached to P(Ph)2 groups coupling with the O-O band. These results provide part of the foundation for understanding oxygen ligands on a silica-supported tetrairidium carbonyl cluster stabilized with bulky electron-donating phosphine ligands [p-tert-butyl-calix[4]arene(OPr)3(OCH2PPh2) (Ph = phenyl; Pr = propyl)]. Reaction of the cluster with O2 also led to the growing in of a Raman band at 788 cm-1, similar to that in the diiridium complex and also assigned to the bonding of a bridging peroxo ligand. Infrared spectra recorded as the supported cluster reacted in sequential exposures to (i) H2, (ii) O2, (iii) H2, and (iv) CO indicate that two bridging peroxo ligands were bonded irreversibly per tetrairidium cluster, replacing bridging carbonyl ligands without altering either the cluster frame or the phosphine ligands. X-ray absorption near edge and infrared spectra include isosbestic points signifying a stoichiometrically simple reaction of the cluster with O2, and mass spectra of the effluent gas show that CO2 formed by oxidation of one terminal CO ligand per cluster as H2 (and not H2O) formed, evidence that hydride ligands had been present on the cluster following treatment (i). The understanding of how O2 reacts with the metal polyhedron provides a foundation for understanding of how oxidation catalysis may proceed on the surfaces of noble metals.

Conformational Stability Effect of Polymeric Iron Chelators.

SummaryThe design and synthesis of metal chelators with extraordinary metal affinities is a basic and challenging scientific problem of both fundamental and practical importance. Here, we demonstrate a “conformational stability effect” that can significantly enhance the metal affinity of ligands after conjugation to polymer chains with the ability to spontaneously adopt a specific conformation as an optimal “soft” scaffold to ensure maximum thermodynamic stability of the metal complexes. Using iron chelators as models, we show that simple conjugation of small molecule catechol ligands to a polyallylamine chain resulted in more than 8–9 orders of magnitude enhancement of the iron-binding affinity, which is comparable to that of enterobactin, the strongest iron chelator ever known. This study demonstrates that flexible polymer chelators may realize the highest possible metal affinities of the conjugated ligands owing to their ability to achieve an optimal conformation, which could advance the identification of strong metal chelators.

Bio-mimetic of catecholase and phosphatase activity by a tetra-iron(III) cluster

An oxido- and acetato-bridged tetranuclear iron(III) cluster, [Fe4III(µ-O)2(µ-OAc)6(phen)2(H2O)2](NO3)2·(H2O)3 (1), [OAc = acetate; phen = 1,10-phenanthroline] has been prepared in the crystalline phase. X-ray structural analysis of the compound reveals that all the Fe(III) centres in 1 adopt an octahedral coordination geometry and the tetra-iron(III) core exists in an unusual asymmetric conformation. Bond valence sum (BVS) calculations recommend the existence of all iron ions in the +3 oxidation state in the crystalline phase. The tetra-iron(III) cluster elegantly catalyzes the oxidation of 3,5-di-tert-butylcatechol (DTBC), viz. catecholase-like activity, with a good turnover number, kcat = 9.28 × 102 h−1 in acetonitrile medium. Spectrophotometric titration shows the existence of two distinct isosbestic points, which unanimously proves the rarely observed enzyme-substrate binding phenomenon in solution. Electrochemical analysis recommends the production of Fe(II)–semiquinone species in the course of the catalytic oxidation of DTBC. Furthermore, the same iron(III) cluster displays phosphoester cleavage activity towards the disodium salt of p-nitrophenylphosphate (PNPP) in aqueous-methanol medium with rate of 7.20 × 10−4 m−1. ESI-MS measurements of the tetra-iron(III) complex in the presence of PNPP indicate the formation of an organophosphorus intermediate in solution and solvent aqua molecules probably make a nucleophilic attack on the phosphorus centre, favouring the generation of the organophosphorus intermediate.

An equation for the isosbestic point wavelength in aqueous solutions of electrolytes

The existence of an isosbestic point in the OH bond vibration band of H2O molecules in solutions of strong electrolytes has been shown by many research groups but its physical meaning has been only recently read as the equilibrium point between two populations of water molecules where either water-ion or water-water interactions respectively prevail. Thus the isosbestic point provides useful information on the bulk displacement of water molecules in presence of solutes. Herein we show that the isosbestic point occurrence is correctly described by an equation describing the balance of the electrostatic forces. Furthermore, its wavelength is also related to the slope of the density vs. the solute concentration. These results support the view that the effect of the solute can be barely regulated within few hydration shells.

Assessment of the VDW interaction converting DMAPS from the thermal-motion form to the hydrogen-bonded form

Assessment of van der Waals (VDW) interactions is fundamental to all of the central quest of structure that regulates the biological function. VDW interactions contributing to intramolecular weak hydrogen bonding are regarded as an important force to regulate the thermal stimuli-sensitive function of sulfobetaine methacrylate, DMAPS. We present here the conversion from the thermal-motion form at room temperature to the weak-hydrogen-bonded form against thermal motion as a terahertz spectral change with a definite isosbestic point from an absorption peak of one form to the other. Vibrational absorptions are used as a probe for assessing VDW interactions in conjunction with highly reliable and well-established density functional theory (DFT) calculations for analysis. Complicated spectral features and uncertain conformations of DMAPS in the amorphous state are clearly resolved under the polarizable continuum model and the dispersion correction for the pure DFT calculations.

Development of an analytical chip for nitrogen monoxide detection using porous glass impregnated with 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl

Nitrogen monoxide (NO) is an important bio-regulatory molecule. NO presence in exhaled breath is a biomarker for an airway inflammation. Thus, measuring NO in the exhaled air is considered a noninvasive method for disease screening. In this study, we developed an analytical chip made of porous glass impregnated with 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO) for NO gas detection. The analytical chip had an absorption peaks at 338 and 567 nm, which decreased when exposed to NO, whereas the absorbance at 413 nm increased with an isosbestic point at 477 nm. It was found that there is a linear relationship between the logarithmic change in the absorbance at 338 nm and 567 nm and the cumulative NO concentration. Additionally, the spectrum of the analytical chip was affected by humidity. There was a linear relationship between the PTIO's molar absorption coefficient at 567 nm and the relative humidity. Therefore, the absorbance of PTIO can be corrected in order to take the effect of humidity into consideration. The working ranges of the analytical chip were between 0.15 and 3.7 ppm × h of cumulative NO concentration and at a relative humidity between 40% and 90%. We also measured NO in a small amount of air (1.8 L to mimic the actual exhaled air volume) and we were able to measure the NO concentration of 30 ppb for 24 h.

Development and Validation of Novel Spectro- Chemometric, Chemometric and TLC-Densitometric Methods for Simultaneous Determination of Timolol and Travoprost in their Bulk Powders and Pharmaceutical Formulation

Simple, sensitive and reliable spectro-chemometric, Chemometric and thin layer chromatography (TLC)-densitometric methods were developed for simultaneous determination of binary mixture of timolol (TIM) and travoprost (TRA) without prior separation in their bulk drugs and dosage forms. The speectro-chemometric methods used are derivative spectrophotometry and isosbestic point methods.

On the Kinetics and Mechanism of the Thiourea Dioxide–Periodate Autocatalysis-Driven Iodine-Clock Reaction

The thiourea dioxide-periodate reaction has been investigated under acidic conditions using phosphate buffer within the pH range of 1.1-2.0 at 1.0 M ionic strength adjusted by sodium perchlorate. Absorbance-time series are monitored as a function of time at 468 nm, the isosbestic point of the I2-I3- system. The profile of these kinetic runs follows either sigmoidal-shaped or rise-and-fall traces depending on the initial concentration ratio of the reactants. The clock species iodine appears after a well-defined but reproducible time lag even in substrate excess, meaning that the system may be classified as an autocatalysis-driven clock reaction. It is also demonstrated that the age of the thiourea dioxide solution markedly shortens the Landolt time, suggesting that the original form of thiourea dioxide (TDO) rearranges into a more reactive form and reacts faster than the original one. The behavior found is consistent with that recently observed in other oxidation reactions of TDO. To characterize the system quantitatively, a 22-step kinetic model is constructed from adapting the kinetic model of the TDO-iodate reaction published recently by supplementing it with six different reactions of periodate. By the help of seven fitted rate coefficients a sound agreement between the measured and calculated absorbance-time traces is obtained.

Temperature dependence analysis of the NIR spectra of liquid water confirms the existence of two phases, one of which is in a coherent state

BackgroundIsosbestic (equal absorption) points in the IR and NIR spectra of liquid water are a well-known feature and they witness the existence of two populations of oscillators in the probed system. Despite it is a well-known experimental fact, in the mainstream molecular dynamics approach the proposed theoretical explanations for it are not able to elucidate which is the physical reason why such a “cut-off frequency” (at the isosbestic point) does exist. MethodsWe investigate pure Milli-Q water on increasing the temperature in the vis-NIR range (400–2500 nm). We specifically payed attention to the first overtone region (1300–1600 nm) of the OH-bond stretching-mode where an isosbestic point has been observed. ResultsA second derivative analysis clearly shows two modes, which can be assigned to water molecules involved in different “hydrogen bonding” configurations whose relative abundance is controlled by the temperature. We have also observed that the ratio of these modes follows a van't Hoff behavior supporting that their energy difference (energy gap) is independent on the temperature. Furthermore, a log-log plot shows a scale invariance of the population ratio with respect to the perturbation (temperature), confirming the existence of a long-range correlated dynamics in the liquid. ConclusionsWe show that the two phases differences between energy and entropy estimated from the experimental data can be compared with the prediction of Quantum Electro Dynamics (QED) showing a remarkable agreement.

Spectroelectrochemical Investigation of 4-Dicyanomethylene 2,6-Dimethyl-4H-Pyran (DDP) Dye with Guanidine Hydrochloride (GuHCl) in Water

Interaction of guanidine hydrochloride (GuHCl) with 4-dicyanomethylene-2,6-dimethyl-4H-pyran (DDP) dye were investigated by photophysical and electrochemical techniques. An enhancement in the fluorescence intensity and formation of an isosbestic point revels that the ground state and excited state properties of DDP dye. The nature of interaction between DDP dye and GuHCl is predominantly through hydrogen-bonding even thoughelectrostatic interaction in aqueous phase operate. The existence of electrostatic interaction by current response function describes that the shift in anodic peak potential towards less positive value indicate electrostatic binding. The existence of more than one microenvironment of DDP dye in aqueous phase is elucidated by Electrochemical Impedence Spectroscopy (EIS) studies through Nyquist plots. These plots signify that there exist at least three different micro environments or dye existing in different fluorescence lifetimes with varying of amplitude. Fluorescence spectral techniques along with electrochemical studies are used as efficient tools to elucidate the nature of interaction of a water soluble probe with hydrogen-bonding solutes is elucidated in our study.

Three-dimensional confocal Raman temperature characterization of electrokinetically pumped microchannels.

A novel method for noninvasive, three-dimensional temperature characterization in microfluidic devices is presented. A specially designed confocal microscope was built and used to measure water temperature by sensing the Raman spectrum variations of the liquid. This is achieved by splitting the spectrum in the isosbestic point and detecting it with two photon counters. The difference between the signals of each detector divided by their sum shows a linear dependence with temperature. A fiber-coupled laser beam is used to pump the sample with 25 mW of optical power at 405 nm. This allows a 0.8 K temperature precision and a 9 μm axial resolution using a 1 s integration time. These features make temperature profiling in all dimensions possible, in contrast with previous methods, where the information present in the height of the channel is lost and the whole spectrum needs to be recovered before computing the sample temperature. Using this technique, different geometries of polydimethylsiloxane microchannels sealed with a 150 μm thick glass coverslip were studied, showing that heat flow through the glass is the dominating dissipation mechanism and defines the maximum temperature in the channel. The results show good agreement with previous work found in the literature.

Controlling the end-to-end assembly of gold nanorods to enhance the plasmonic response in near infrared

In the current paper, we report an efficient approach to produce in a controllable manner electromagnetic hot-spots localized in linear assemblies of gold nanorods (AuNRs) that can be further exploited as highly active surface-enhanced Raman scattering (SERS) sites. The process of the AuNRs self-assembling was initiated in an aqueous solution at pH = 3.1 in the presence of L-cysteine (Cys) molecules and was monitored in real-time for minutes up to hours. The recorded sequential extinction spectra featuring a well-defined isosbestic point at 834 nm is consistent with the first-order like reaction between individual and end-to-end assembled AuNRs as final products. Moreover, the dynamic of the self-assembling process was examined by correlating the extinction spectra with the average number of AuNRs in the chains as provided from the transmission electron microscopy (TEM) pictures. Noteworthy, the average number of connected AuNRs can be fixed at any moment in solution, depending of the desired plasmonic response, by blocking the assembling process with a negative layer of poly (sodium-p-styrenesulfonate) (PSS) polyelectrolyte. The high electric field in the hot-spots in between the linked nanoparticles was proved by SERS experiments using as target analyte para-aminothiophenol (p-ATP) and conducted to clear the demonstration of a higher enhancement factor relative to the individual AuNRs. Finally, all experimental findings and, especially, the far and near-field optical properties of such fabricated plasmonic nanoassemblies were supported by finite-difference time-domain (FDTD) simulations.